SpeedFan - Hardware monitor for Windows that can access digital temperature sensors located on several 2-wire SMBus Serial Bus. Can access voltages and fan speeds and control fan speeds. Includes technical articles and docs.
smcFanControl - [Download] smcFanControl 2.
Argus Monitor - Argus Monitor is for monitoring and analyzing the temperature and the health status of the hardware parts of the system.
Open Hardware Monitor - Monitors temperature sensors, fan speeds, voltages, load and clock speeds, with optional graph.
Macs Fan Control - Macs Fan Control is an iMac, MacBook Pro, MacBook Air, Mac Mini and Mac Pro fan control that is the solution of two basic issues namely noise problems and overheating problems.
TG Pro - TG Pro is basically a tuner for the Mac OS X that provides the in-depth analysis of the Mac system regarding fan position, fan controlling, fan monitoring and detailed internal temperatures.